Production of electrical elements



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United States Patent PRODUCTION OF ELECTRICAL ELEMENTS Peter Paul Hopf,London, and Ronald Edgar John Lish- No Drawing. Application November 10,1949,

Serial No. 126,701

Claims priority, application Great Britain November 12, 1948 13 Claims.(Cl. 117-212) This invention relates to the production of printedelectrical components and printed circuits.

During the past few years an increasing demand for printed circuits hasarisen as their potentialities have been more fully appreciated and themethods of producing them have improved.

Several methods for producing printed circuits have been described. Oneis by painting the circuit on to the base material using a suitablypigmented paint: the painting may be done through a stencil. A variantthereof is to spray molten metal. Deposition of metal through a stencilas a result of simultaneously spraying separately prepared solutions ofsuitable reacting chemicals has also been used. Metal spattering throughstencils has been proposed.

In yet another type of process a foil of metal is bonded on to the basematerial and after applying a resist to those parts of the metal surfacein which a conductor is required the remaining metal is etched away andthe resist then removed. The usual bonding material em: ployed is athermo-setting adhesive. A variant of this is a transfer process using apaper coated with pigment and resin, involving a hot stamping operation.

in operating any process of this character it is important to obtaina'uniform product with a minimum of rejects and in most of the knownprocesses it has been found that there are various difficulties whichlead to lack of uniformity and consequently the proportion of rejects isfairly high.

According to the present invention there is provided a process forproducing a printed electrical element which comprises applying to thesurface of an organic electrically insulating material a pattern of anelectrical conducting path as a coating, substantially free from anypermanent binder, of an at least low electrically conducting material infinely divided form and thereafter simultmleously applying heat andpressure to at least the coated areas whereby the individual particlesof conducting material cohere together to form said conducting path atleast partially embedded in the underlying insulating material.

The start ng materials employed in the present process are a finelydivided form of the conductor such as finely divided silver or carbon.This is made up into a slurry in a volatile diluent to which may beadded a small proportion of a less volatile organic solvent which mayact as a temporary adhering agent. Examples of suitable diluents are thelower alkanols, especially methanol and ethanol, the cyclic ethers suchas dioxane, the lower aliphatic ketones, such as acetone and methylethyl ketone, and water. In selecting the diluent regard must be had forany undesirable action, such as a swelling action, which it might haveupon the surface of the insulating material. The volatile liquid isrecited in the claims as inert to the plastic, that is as exerting nochemical, swelling or other objectionable action upon the plastic.Typical examples of conductors useful in the process are finely dividedsilver obtained by filtering and washing ice the composition preparedaccording to the copending application Serial No. 777,116, filedSeptember 30, 1946, and now Patent 2,592,870, finely divided copper,carbon black and colloidal graphite. Examples of less volatile organicdiluents which may be present in the slurries are the lower polyhydricalcohols, such as ethylene glycol and glycerine and the amino-alcohols,such as monoethanolamine. The proportion by weight of solids in theslurry varies greatly depending upon the material. In the case of finelydivided silver the proportion may be about 4 to 1 whilst with carbonblack 1 to 1 is suitable.

The organic electrically insulating material may be a thermoplastic orthermoset material or it may be a high polymer. It may also be loadedwith inorganic insulating materials such as asbestos, silica or glassfibres. Examples of suitable materials are electrical grades ofphenol-formaldehyde and aniline-formaldehyde synthetic resins,polyacrylic resins, such as polymethyl methacrylate, polyvinyl resins,such as polystyrene and polythene. These materials may be used in avariety of forms such as sheets, films, plates and blocks. A film may becarried upon a supporting base and may be produced thereupon by coating.Such film may be more highly polymerized or condensed to the form havingthe most favourable electrical properties by the action of heat oractinic light. Such polymerization or condensation may be carried out atany convenient stage after coating the film. Such step may be carriedout in two or more stages i. e. a partial curing prior to applying theconductor followed by a second partial curing after the conductor hasbeen set.

In describing one form of the invention it will be assumed that it isdesired to produce a printed circuit in silver upon aphenol-f0rmaldehyde sheet material. A stencil is prepared Whose cut outportions correspond to the outline of the circuit. The stencil isapplied to one surface of the sheet material, which may first have hadany holes that are desired punched into it. The stencil-covered surfaceis then sprayed with a slurry of sponge silver in methanol and thestencil then removed leaving a pattern corresponding to that of theprinted circuit that it is desired to produce. The greater part of themethanol is then removed by drying, a small amount being retained foradhering the silver to the sheet. The sheet is then placed in a presswhich has a heated platen and the platen is then brought into contactwith the surface carrying the pattern. It is preferred to apply themaximum possible pressure and to use the lowest possible temperature inthis operation, and in general times of the order of 1-10 seconds areused. With a phenol-formaldehyde insulating material a pressure of about1000 lbs/in. at about 200 C. and a contact time of 4 seconds has beenfound suitable.

Instead of using a press the sheet may be passed between a pair ofrollers, the one in contact with the coated surface being heated. Thepressure at the nip of the rollers should be adjustable to any desiredvalue. It is preferred to apply the maximum possible pressure and thelowest possible temperature in this operation. If desired, the coatedsurface can be preheated, as by radiant heat, immediately prior topassing to the rollers, but if this is done it may be necessary todisplace the atmosphere in contact with the coated surface so as toprevent oxidation: nitrogen is suitable for this purpose.

With thermoset materials quite high temperatures, such as 200250 C., andhigh pressures such as 10002000 lbs/in. are preferred. Materials whichsoften on heating may be printed using a plate maintained at a muchhigher temperature than that at which they first soften.

When using a thermoplastic material or a material like polythene whichhas a definite and rather low melting are applied.

point it is preferred, prior to applying heat and pressure, to interposea thin sheet of paper which absorbs organic liquids and provides a pathfor the escape of vapours as well as protecting areas to which heat andpressure is not to be applied. 7

The pattern of the component or circuit need not be produced on theinsulating material using a stencil. Other methods, such as offsetprinting may be used.

When a printed circuit has been produced in the manner described abovethe spaces that have been left for resistors can be overprinted. Theprocedure already de' scribed is then repeated using a second stencilwhose cutout portions correspond to the outline of the resistors. Thisstencil is applied, in register, to the once printed sheet and agraphite slurry then sprayed on. The cut-out portions should provide forslight overlap onto the conductors at the appropriate points in order toprovide contacts. The further procedure is the same as for printing thesilver circuit.

7 The above procedure may be modified by separately printing the outlineof the contacting circuit and of one or more resistances in the circuitand the first heat and pressure step can then be omitted. It is alsopossible to reverse the procedure above outlined by first printingtheresistors and then using a second stencil the outline of whichcorresponds to that of the required printed circuit.

Not only complete circuits but specific components such as coils can beprinted in the manner described.

The invention can be used to produce various electrical components suchas coils and resistances. Particular interest attaches to the productionof resistances since these can be made so as to cut out portions of anydesired resistance.

The value of a resistance may be expressed as .L WT

variations in the value of T will considerably afiect the value'of R.Moreover to produce a satisfactory resistance p and T should be constantthroughout and the value of the resistance will then depend only on itslength and width.

The present invention enables printing methods such r as offset printingto be employed and in these substantially uniform films are employed.Thus a metal block may be prepared'of the pattern of the resistance, forexample, a grid system. The block is inked with the slurry 7 based onfinely divided low conducting material such as graphite, and diluentsubstantially free from permanent binder. The image on the block istransferred to a rubber blanket from which it is; in turn, transferredto the plastic material. The resulting films are subject to heat andpressure treatment to form the finished resistance which has a definitefixed resistance per square. Circuit lay out will usually determine themaximum length, but, by varying the width, a resistance of desired valuemay be cut for insertion in a circuit and fixed into position with anadhesive or by riveting.

The present process requires that the conductor b applied to the surfaceof the insulating material without a permanent binder being present. Asmall amount of higher boiling organic liquid or the residue of thevolatile diluent being relied upon to maintain a sufficient adherence ofthe particles of the conductor to the insulating material and this isvolatilised when heat and pressure In the product the particles firmlycohere to each other and at least the lower layers thereof are finelyembedded in the insulating material.

The higher boiling organic liquid may be so chosen as to counteract anytendency of the conductor to undergo chemical change in the earlierstages of the process. Thus when sponge silver is used a small additionof an ethanolamine may be made to prevent oxidation to silver oxide orto reverse such change, ifit has occurred, during the hot pressing.However since the reduction involved is an exothermic process it is notpossible to use silver oxide per se in the coating step.

Difierent circuits may be printed upon the two sides of the insulatingmaterial. The second side may have an electronic shield printed thereon.

The following examples illustrate the manner in which the invention maybe carried into effect:

Example 1 A block is prepared of a circuit to be printed and mounted onan otf-set press having a rubber blanket for transfer. The block isinked with a slurry consisting of 70% by weight of finely dividedsilver, 20% of triethanolamine alginate and 10% of ethylene glycol; Theslurry is transferred in the usual way for oil-set printing on to asheet of polystyrene. The print is now sprinkled with finely dividedsilver, then with French chalk and brushed with a sable brush. Theprinted sheet is covered with a sheet of very fine rag paper andinserted in a press, the platen of which is run at C. and the pressurebeing about 500 lbs/in. and pressed for 0.5 second. The sheet is thenremoved from the press and is washed with water.

Example 2 A pattern of a printed circuit is applied to an electricalgrade of a phenol-formaldehyde laminate by the silk screen method usinga slurry containing 70% by weight of finely divided silver, 5 ofmonoethanolamine and 25% of methanol. The print is partially dried byinfra-red heating, placed in a press and subjected to a temperature of210 C. and 1000 lbs/in. for 4 seconds. The laminate is removed andwashed.

To apply the resistors to this circuit it is printed a second time bythe silk screen process the stencil employed having been cut out toleave the resistor areas uncovered and allow slight overlap on to theappropriate portions of the conductors. The slurry used is 20% by weightof carbon black, 5% of glycerine and 75% of methanol. The print ispartially dried, placed in the press, covered with a sheet of very finerag paper and then given the same temperature, pressure and timetreatment as previously. The sheet is then taken out of the press andwashed.

Example 3 A letter press block is prepared of a circuit to be printedand this is mounted on an oif-set press, having a rubber blanket fortransfer. The block is inked with a slurry consisting of 70% by weightof finely divided silver, 20% of triethanolamine alginate and 10% ofethylene glycol. The slurry is transferred in the usual way for off-setprinting on to a sheet of polystyrene. The print is now sprinkled withfinely divided silver and dusted with a sable brush which has previouslybeen dipped in French chalk.

The printed sheet is covered with a sheet of very fine rag paper andinserted in a press, the platen of which is heated to 150 C., thepressure being 500 lbs./sq. in. and then pressed for 0.5 second.

Example 4 seconds the circuit is bonded to the laminate in the patterndesired.

Example 5 In Example 4 instead of applying pressure to the wholesurface, a heated die having the shape of the pattern required ispressed, in register, on to the surface of the laminate. Any silverwhich is not fixed may be removed by washing.

Example 6 A silicone laminate is coated with a vinyl chloride copolymeridentified as Bakelite J. 11185 in the areas to which it is desired toapply the resistors. All solvent is allowed to evaporate therefrom so asto produce a nontacky film of copolymer upon a silicone support. To thecopolymer areas there is applied, by means of spraying through astencil, a slurry consisting of 50% by weight of finely divided carbon,40% of ethanol and of glycerine.

The laminate is placed in a press and subjected to a temperature of 180C. and pressure of 1,000 lbs/sq. in. for 10 seconds. It is thentransferred to an oven at 150 C. for 10 minutes to completely cure thecopolymer.

Example '7 An aniline-formaldehyde resin filled with paper fibres iscoated with a urea-formaldehyde varnish solids in butanol-toluenemixture) and infra-red heating applied to evaporate off all solvent andto partially cure the urea-formaldehyde condensation product present inthe varnish. Through a stencil resistances are sprayed by methoddescribed in Example 6. The resistances are dried by infra red heating.

The stencil is removed and a stencil of the required circuit is fittedto the laminate and a circuit sprayed as described in Example 4 andpartially dried by infra-red heating.

The laminate is placed in a press, the platen of which is heated to 180C. and a pressure of 500 lbs/sq. in. applied to the whole laminate for10 seconds. The laminate with the resistors and circuit produced isplaced in an oven for 10 minutes at 150 C. to completely cure thecondensation product.

Example 8 A pattern of a printed circuit is applied to an electricalgrade of a phenol-formaldehyde laminate by the silk screen method usinga slurry containing 70% by weight of finely divided silver, 5% ofmonoethanolamine and of methanol. The print is partially dried byinfra-red heating, placed in a press and subjected to a temperature of210 C. and 1,000 lbs/sq. in. for 4 seconds.

To the circuit thus produced the resistors are applied by sprayingfirstly the vinyl chloride copolymer of Example 6 in a volatile solvent,drying and spraying a slurry of by weight of carbon, of ethanol and 10%of monoethanolamine through a stencil cut to the pattern of the desiredresistances. The laminate is placed in a press and subjected to apressure of 1,000 lbs./ sq. in. and temperature of 180 C. for 10seconds. The laminate is transferred to an oven and the copolymercompletely cured.

Example 9 A sheet of silicone treated glass fibre is coated withcopolymer as described in Example 6. A slurry of 20% by weight of finelydivided carbon, of ethanol and 15% of glycerine, is sprayed through astencil of the required resistor pattern. The laminate is placed in apress and subjected to a temperature of 180 C. and pressure of 1,000lbs/sq. in. for ten seconds. The laminate is transferred to an oven andthe copolymer cured.

To apply the circuit to these resistors a slurry of by weight of finelydivided silver, 5% of monoethanolamine and 25% of methanol is sprayedthrough a stencil cut in the pattern of the required circuit. Thecoating is partially dried by infra-red heating and placed in a press.

A die, in the circuit pattern desired, heated to C. and accuratelyregistered, is applied under a pressure of 1,000 lbs/sq. in. for 4seconds. The laminate is then placed in an oven for a further 10 mintuesat 150 C. to complete the curing of the copolymer.

We claim:

1. A process for imprinting a circuit element upon a substantiallysmooth surface of electrical insulating synthetic plastic material,which comprises applying to selected areas of such plastic material, aslurry of finely divided circuit element forming particles in a volatileliquid that is inert to the plastic, and, prior to complete evaporationof the volatile liquid, applying mechanical pressure upon the slurrycovered plastic of the order of 500 to 2000 pounds per square inch andheat of the order of 150 to 250 C. for a period of time of the order of0.5 to 10 seconds, with resultant complete evaporation of the volatileliquid, to thereby secure embedding of the circuit element formingparticles in the surface of the plastic material and coherence of suchparticles without substantially impairing the electrical properties ofthe surface of said material.

2. The combination recited in claim 1 in which the circuit elementforming particles are of silver.

3. The combination recited in claim 1 in which the circuit elementforming particles are of carbon black.

4. The combination recited in claim 1 in which the volatile liquid isselected from the group consisting of water, the lower alkanols, thelower cyclic ethers, the lower aliphatic ketones, and mixtures thereof.

5. The combination recited in claim 4 in which the volatile liquidincludes a component of lower volatility to retain the solid particlesin place on the plastic material prior to application thereto of thebonding pressure and heat, said component of lower volatility beingselected from the group consisting of the lower polyhydric alcohols andthe amino-alcohols.

6. The combination recited in claim 1 in which two sets of circuitforming elements are applied to the plastic, one set being highlyconductive elements composed of metal particles, the other set beingresistor elements composed of carbon black particles, and in whichcontinuity of circuit between conductive and resistor elements ismaintained by end overlapping of the areas of slurry applied.

7. The combination recited in claim 6 in which the pressure and heatstep is performed on the first slurry application before the applicationof the second slurry.

8. The process recited in claim 1, applied for forming two types ofcircuit forming elements on the same plastic surface, consisting of aset of conductive elements and a set of resistor elements, in which theslurry of one category of circuit elements is applied in one step andthe circuit elements of the other type of circuit elements are appliedin a second step, with end overlap of slurry areas for continuity ofcircuit, and in which the pressure and heat for efifecting embedding andcohesion of the respective particles of each category and completevolatilization of the liquid are efiEected in a single pressure and heatapplication after both categories of slurry have been applied.

9. The combination recited in claim 1 in which the plastic material isof thermoplastic character of relatively low melting point and in whichthe mechanical pressure is applied to the slurry covered plastic throughan interposed absorbent sheet for effective escape of vapors andprotection of the plastic during pressure and heat application.

10. A process for imprinting a circuit element upon a substantiallysmooth surface of electrical insulating synthetic plastic material whichcomprises applying to a selected area of said plastic material a slurryof finely divided highly conductive element forming particles in avolatile liquid that is inert to the plastic and a slurry of finelydvided resistor element forming particles in a volatile liquid that isinert to the plastic, portions of the two slurries overlapping tomaintain continuity therebetween,

and, prior to completion of volatilization of the volatile liquids,simultaneously applying mechanical pressure and heat to the slurrycovered plastic to complete volatilization of said'volatile' liquids andto cause said particles to cohere and to become at least partiallyembedded in the V underlying insulating material without substantiallyimpairing the electrical properties of the surface of said material.

' 11. The process as claimed in claim in which the pressure and heat aresimultaneously applied to the first slurry to complete volatilization ofthe volatile liquid and to cause the particles in the first slurry tocohere and to become at least partially embedded in the underlyinginsulating material before the application of the second slurry, andafter application of the second slurry the pressure and heat aresimultaneously applied to the second slurry to complete volatilizationof the volatile liquid and to cause the particles in said second slurryto cohere and to become at least partially embedded in the underlyinginsulating material.

12. The process as claimed in claim 10 in which the I finely dividedhighly conductive element forming particles are metallic particles, andthe resistor element forming particles are carbon black particles.

13. A process for imprinting a circuit element upon a substantiallysmooth surface of electrical insulating thermoplastic material having alow melting point, which comprises applying to a selected area of saidplastic material a slurry of finely divided circuit element formingparticles in a volatile liquid that is inert to the plastic, placing anabsorbent sheet over said plastic material and applied slurry, and priorto completion of volatilization of the volatile liquid, simultaneouslyapplying mechanical pressure and heat to the absorbent sheet to causethe individual particles to cohere and to become at least partiallyembedded in the underlying insulating material without substantiallyimpairing the electrical properties of the surface of said material,said interposedabsorbent sheet promoting eifective escape of vapors andprotecting the plastic during the application of pressure and heat.

References Cited in the file of this patent UNITEDSTATES PATENTS 122,636Muller Jan. 9, 1872 1,717,193 Dantsizen June 11, 1929' 1,837,678 RyderDec. 22, 1931 1,922,254 McCulloch Aug. 15, 1933 1,987,969 Parkin Jan.15, 1935 2,056,928, Magdziarz Oct. 6, 1936 2,060,114 Podolsky Nov. 10,1936 2,121,005 Bener June 21, 1938 2,136,370 Bock ius et al. Nov; 15,1938 2,158,552 Potdevin May 16, 1939 2,177,484 Fruth Oct. 24, 19392,252,776 Losee Aug. 19, 1941 2,373,676 Germeshavsen Apr. 17, 19452,397,827 Williams Apr. 2, 1946 2,429,089 Box Oct. 14, 1947 2,441,960Eisler May 25, 1948 2,473,183 Watson June 14, 1949 2,474,988 SargroveJuly 5, 1949 2,492,429 John Dec. 27, 1949 FOREIGN PATENTS 269,729 GreatBritain Apr. 28, 1927 OTHER REFERENCES

1. A PROCESS FOR IMPRINTING A CIRCUIT ELEMENT UPON A SUBSTANTIALLYSMOOTH SURFACE OF ELECTRICAL INSULATING SYNTHETIC PLASTIC MATERIAL,WHICH COMPRISES APPLYING TO SELECTED AREAS OF SUCH PLASTIC MATERIAL, ASLURRY OF FINELY DIVIDED CIRCUIT ELEMENT FORMING PARTICLES IN A VOLATILELIQUID THAT IS INERT TO THE PLASTIC, AND, PRIOR TO COMPLETE EVAPORATIONOF THE VOLATILE LIQUID, APPLYING MECHANICAL PRESSURE UPON THE SLURRYCOVERED PLASTIC OF THE ORDER OF 500 TO 2000 POUNDS PER SQUARE INCH ANDHEAT OF THE ORDER OF 150 TO 250* C. FOR A PERIOD OF TIME OF THE ORDER OF0.5 TO 10 SECONDS, WITH THE RESULTANT COMPLETE EVAPORATION OF THEVOLATILE, LIQUID, TO THEREBY SECURE EMBEDDING OF THE CIRCUIT ELEMENTFORMING PARTICLES IN THE SURFACE OF THE PLASTIC MATERIAL AND COHERENCEOF SUCH PARTICLES WITHOUT SUBSTANTIALLY IMPAIRING THE ELECTRICALPROPERTIES OF THE SURFACE OF SAID MATERIAL.